Er3+ doped boro-tellurite glasses for 1.5 mum broadband amplification

ABSTRACT

A tellurite-based glass composition for use in EDFAs exhibits higher phonon energy without sacrificing optical, thermal or chemical durability properties. The introduction of boron oxide (B 2 O 3 ) into the Er 3+ -doped tellurite glasses increases the phonon energy from typically  785  cm −1  up to  1335  cm −1 . The inclusion of additional glass components such as Al 2 O 3  has been shown to enhance the thermal stability and particularly the chemical durability of the boro-tellurite glasses. Er:Yb codoping of the glass does further enhance its gain characteristics.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] This invention relates to Er³⁺ doped tellurite glasses and morespecifically to Er³⁺ doped boro-tellurite glasses with increased phononenergy for 1.5 μm broadband amplification.

[0003] 2. Description of the Related Art

[0004] Optical amplifiers are considered enabling components forbandwidth expansion in fiber optic communications systems. Inparticular, silica glass erbium doped fiber amplifiers (EDFA) exhibitmany desirable attributes including high gain, low noise, negligiblecrosstalk and intermodulation distortion, bit-rate transparency, andpolarization insensitive gain. These properties make optical fiberamplifiers superior to semiconductor devices as amplifiers in fiberoptic systems. Moreover, fiber-based amplifiers do not requireconversion from electrical energy to photon energy. In a communicationssystem of any significant size, there is typically a distributionnetwork that includes long communication paths and nodes where thenetwork branches. In such a network, amplifiers are required in order tomaintain the amplitude of the signal and the integrity of any data inroute between a source and destination. To function properly, theamplifiers must exhibit high small signal gains and/or high outputsaturation powers over a desired bandwidth. One drawback of silica EDFAsis their limited 30 nm bandwidth, which limits the transmission capacityof WDM systems.

[0005] Tellurite glasses provide a broad bandwidth of over 70 nm andthus have received considerable attention for use in EDFAs. See A. Moriet al “1.5 μm Broadband Amplification By Tellurite-Based EDFAs,”Technical Digest of Conf. Optical Fibe-Comm. 1997 (OFC′97), Feb 16-21,1997 and Y. Ohishi et al. “Gain Characteristics of Tellurite-BasedErbium-Doped Fiber Amplifiers for 1.5 μm Broadband Amplification” Opt.Lett., vol. 23, no. 4, 1998, p. 274.

[0006] To amplify a 1.5 μm signal, EDFAs can be optically pumped at 1480nm or at 980 nm as shown in the energy level diagram 10 of Er³⁺, FIG. 1.Pumping at 1480 nm is typically used for high power EDFAs because theground state absorption to the ⁴I_(13/2) energy level has a highabsorption cross-section relative to the ⁴I_(11/2) energy level.Unfortunately, this scheme does not provide full population inversion orgood SNR and is not adequate for many EDFA applications.

[0007] 980 nm optical pumping provides good SNR and low cost but thesmall signal gain is significantly less than what is achieved with 1480nm pumping for erbium doped low phonon energy glass fibers, such asfluorite glass fiber and tellurite glass fiber. Tellurite glassescontain heavy elements, which translates into small phonon energy(typically between 680 and 785 cm⁻¹) as compared to silicate glasseswhich present high phonon energy (typically around 1100 cm⁻¹). Phononenergy has a strong influence on the lifetimes of the different excitedstates of Er³⁺ because the relaxation between levels is dominated bymultiphonon processes. The larger the number of phonons involved, thesmaller the probability of relaxation to the lower energy level, and thelonger the lifetime of a given excited state, for instance ⁴I_(11/2) oferbium ions.

[0008] With 980 nm pumping the level ⁴I_(11/2) gets populated first, andthen through phonon-assisted relaxation the lower level ⁴I_(13/2) getspopulated. Gain is achieved through transition between the levels⁴I_(13/2) and ⁴I_(15/2). For optimal operation, the lifetime of thelevel ⁴I_(11/2) should be as short as possible. Otherwise, excited stateabsorption processes from the level ⁴I_(11/2) to higher energy excitedstates such as ⁴F_(7/2) will occur and reduce the gain at 1550 nm.Consequently, the low phonon energy of tellurite glass creates longerlifetimes, which in turn reduces small signal gain when pumped with 980nm laser diode.

[0009] Y. G. Choi et al, “Enhanced ⁴I_(11/2)→⁴I_(13/2) Transition Ratein Er³⁺/Ce³⁺-Codoped Tellurite Glasses,” Electron. Lett. Vol. 35, no.20, 1999, p. 1765 proposed Ce³⁺-codoping to enhance the 980 nm pumpingefficiency through the non-radiative energy transfer Er³⁺:⁴I_(11/2),Ce³⁺:²F_(5/2)→Er³⁺:⁴I_(13/2,) Ce³⁺:²F_(7/2). The co-doping provides anadditional channel for the relaxation ⁴I_(11/2→) ⁴I_(13/2) in theEr³⁺-doped tellurite glasses which shortens the lifetime of the⁴I_(11/2) level and enhances the population accumulation in the⁴I_(13/2) level and the 980 nm pumping efficiency.

[0010] A more effective approach would be to increase the phonon energyof the tellurite glass without sacrificing the glass' optical, thermalstability or chemical durability properties.

SUMMARY OF THE INVENTION

[0011] The present invention provides a tellurite-based glasscomposition for use in EDFAs that exhibits higher phonon energy withoutsacrificing optical, thermal stability or chemical durabilityproperties.

[0012] This is accomplished by introducing boron oxide (B₂O₃), which hasa phonon energy up to 1335 cm⁻¹, into the Er³⁺-doped tellurite glasses.The introduction of B₂O₃ increases the phonon energy of the host glassand the multiphonon relaxation rate of the ⁴I_(11/2)→⁴I_(13/2)transition, which enhances the population accumulation in the ⁴I_(13/2)level and the 980 nm pumping efficiency. The inclusion of additionalglass components such as Al₂O₃ has been shown to enhance the thermal,stability and particularly the chemical durability of the boro-telluriteglasses. Er:Yb codoping of the glass will further enhance its pumpefficiency

[0013] In one embodiment, the boro-tellurite glass composition for thefiber core includes the following ingredients: a glass network formerTeO₂ from 50 to 70 mole percent, B₂O₃ from 5 to 22 mole percent, A₂O₃from 5 to 18 mole percent, a glass network modifier R₂O from 5 to 25mole percent, a glass network modifier MO from 0 to 15 mole percent,GeO₂ from 0 to 7 mole percent and rare-earth dopant L₂O₃ from 0.25 to 10weight percent wherein R₂O is selected from oxides Li₂O, Na₂O, K₂O andmixtures thereof, MO is selected from oxides MgO, CaO, BaO, ZnO andmixtures thereof, A₂O₃ is selected from Al₂O₃, Y₂O₃ and mixturesthereof, and rare-earth dopant L₂O₃ is selected from rare earth oxidesEr₂O₃, Yb₂O₃, Tm₂O₃, Tb₂O₃, CeO₂, Sm₂O₃ and Nd₂O₃ and mixtures thereof.The cladding glass has a similar composition absent the rare-earthdopants.

[0014] In another embodiment, the boro-tellurite glass composition forthe fiber core includes the following ingredients: a glass networkformer TeO₂ from 55 to 65 mole percent, B₂O₃ from 10 to 20 mole percent,A₂O₃ from 7 to 15 mole percent, a glass network modifier R₂O from 10 to20 mole percent, a glass network modifier MO from 0 to 10 mole percent,GeO₂ from 0 to 5 mole percent and rare-earth dopant L₂O₃ from 0.25 to 6weight percent. In one embodiment, the glass comprises Al₂O₃ (A₂O₃) from7 to 15 mole percent and Na₂O (R₂O) from 10-20 percent. In anotherembodiment, the glass comprises Al₂O₃ from 10 to 15 mole percent. Theglass may be doped with, for example, 0.25 to 3 wt. % percent Er₂O₃,0.25 to 5 wt. % of an Er₂O₃ and Yb₂O₃ mixture, 0.25 to 5 wt. % each ofEr₂O₃ and Yb₂O₃, or approximately 0.25 to 3 wt. % of Tm₂O₃.

[0015] These and other features and advantages of the invention will beapparent to those skilled in the art from the following detaileddescription of preferred embodiments, taken together with theaccompanying drawings, in which:

BRIEF DESCRIPTION OF THE DRAWINGS

[0016]FIG. 1, as described above, is the energy level diagram of Er³⁺ intellurite glass;

[0017]FIGS. 2 through 4 provide boro-tellurite glass compositions inaccordance with the present invention;

[0018]FIG. 5 is a diagram of a boro-tellurite based EDFA;

[0019]FIGS. 6 and 7 are differential scanning calorimetry (DSC) curvesillustrating the thermal stability of the boro-tellurite glass;

[0020]FIGS. 8 through 10 are plots of weight loss per unit of surfaceillustrating the chemical durability of the boro-tellurite glass;

[0021]FIGS. 11 through 14 are transmission spectra illustrating theincreased phonon energy of the boro-tellurite glass; and

[0022]FIGS. 15 and 16 are gain curves and gain spectra illustrating theenhanced gain of a boro-tellurite glass fiber.

DETAILED DESCRIPTION OF THE INVENTION

[0023] The present invention provides a tellurite-based glasscomposition for use in EDFAs that exhibits higher phonon energy withoutsacrificing optical, thermal stability or chemical durabilityproperties. Boron oxide (B₂O₃), which has a phonon energy up to 1335cm⁻¹, is introduced into the Er³⁺-doped tellurite glasses. Theintroduction of B₂O₃ increases the phonon energy of the host glass andthe multiphonon relaxation rate of the ⁴I_(11/2)→⁴I_(13/2) transition,which enhances the population accumulation in the ⁴I_(13/2) level andthe 980 nm pumping efficiency. The inclusion of additional glasscomponents such as Al₂O₃ has been shown to enhance the thermal stabilityand particularly the chemical durability of the boro-tellurite glasses.

[0024] In glass compositions, the glass network former, modifier andother elements are typically specified in mole % because the glassstructure is related with the mole % of every element in the glass. Thedopants are typically specified in weight % because the dopingconcentration in term of ions per volume, e.g., ions per cubiccentimeters, can be readily derived and is critical information forphotonic and optical related applications.

Boro-Tellurite Glass Composition

[0025] As shown in FIG. 2, the boro-tellurite glass composition 20 forthe fiber core includes a glass network former TeO₂ from 50 to 70 molepercent. TeO₂ concentrations in excess of 70 mole percent produceglasses with a strong tendency to crystallize. B₂O₃ from 5 to 22 molepercent is introduced into the tellurite glass to raise the phononenergy of the lattice. Concentrations in excess of 22 mole percent tendto cause phase separation in the glass. The glass includes A₂O₃ (Al₂O₃,Y₂O₃ and mixtures thereof) from 5 to 18 mole percent, to increase theglass transition temperature and improve thermal stability andparticularly chemical durability. When the content of A₂O₃ exceeds 18mole percent, the melting temperature of glass becomes too high anddecomposition and crystallization could occur. The glass compositionfurther includes network modifiers R₂O (oxides Li₂O, Na₂O, K₂O andmixtures thereof) from 5 to 25 mole percent and MO (oxides MgO, CaO,BaO, ZnO and mixtures thereof) from 0 to 15 mole percent. The networkmodifiers are needed to obtain a stable tellurite glass but tend todeteriorate its chemical durability and lead to crystallizationappearance when present at elevated concentrations. GeO₂ from 0 to 7mole percent may be added to increase the glass transition temperatureand refractive index and improve thermal stability but GeO₂ is anexpensive material. Finally, the core glass is doped with a rare-earthdopant L₂O₃ from 0.25 to 10 weight percent selected from rare earthoxides Er₂O₃, Yb₂O₃, Tm₂O₃, Tb₂O₃, CeO₂, Sm₂O₃ and Nd₂O₃ and mixturesthereof. The cladding glass has a similar composition absent therare-earth dopants. The introduction of Al₂O₃ into the glass has beenfound to be particularly effective at improving chemical durability.

[0026] As shown in FIG. 3, a boro-tellurite glass composition 30 for thefiber core includes the following ingredients: a glass network formerTeO₂ from 55 to 65 mole percent, B₂O₃ from 10 to 20 mole percent, A₂O₃from 7 to 15 mole percent, a glass network modifier R₂O from 10 to 20mole percent, a glass network modifier MO from 0 to 10 mole percent,GeO₂ from 0 to 5 mole percent and rare-earth dopant L₂O₃ from 0.25 to 6weight percent. In one embodiment, the glass comprises Al₂O₃ (A₂O₃) from7 to 15 mole percent and Na₂O (R₂O) from 10-20 percent. In anotherembodiment, the glass comprises Al₂O₃ from 10 to 15 mole percent. Theglass may be doped with, for example, 0.25 to 3 wt. % percent Er₂O₃,0.25 to 5 wt. % of an Er₂O₃ and Yb₂O₃ mixture, 0.25 to 5 wt. % each ofEr₂O₃ and Yb2O₃, or approximately 0.25 to 3 wt. % of Tm₂O₃.

[0027] As shown in FIG. 4, a boro-tellurite glass composition 40 for thefiber core includes the following ingredients: a glass network former ofapproximately 60 mole percent TeO₂, approximately 15 mole percent B₂O₃,approximately 10 mole percent Al₂O₃, and approximately 15 mole percentNa₂O. The glass may be doped with, for example, 0.25 to 3 wt. % percentEr₂O₃, 0.25 to 5 wt. % of an Er₂O₃ and Yb₂O₃ mixture, 0.25 to 5 wt. %each of Er₂O₃ and Yb₂O₃, or approximately 0.25 to 3 wt. % of Tm₂O₃.

[0028] As previously described, in typical tellurite glass the lowphonon energy causes the lifetime of the ⁴I_(11/2) level of erbium ionsto be relatively long, which in turn reduces small signal gain. Inaddition, the long lifetime lowers the transfer efficiency from Yb ionsto Er ions so co-doping is not beneficial, hence not used in knowntellurite glasses.

[0029] The introduction of boron oxide into the tellurite glassincreases the phonon energy, which, as described above, has a directimpact on small signal gain. In addition, the higher phonon energyreduces the lifetime of the ⁴I_(11/2) level. This shortening of thelifetime reduces back energy transfer from Er to Yb ions and makes theEr:Yb codoping beneficial. Thus in certain cases the glass is co-dopedwith Er:Yb to increase the pump efficiency of the glass, fiber and EDFA.

[0030] Co-doping with ytterbium enhances population inversion of theerbium ⁴I_(13/2) metastable state. The Yb³⁺ excited states ²F_(5/2) arepumped from the Yb³⁺ ²F_(7/2) ground state with the same pump wavelengththat is used to excite upward transitions from the erbium ground state⁴I_(15/2). Energy levels of the excited ytterbium ²F_(5/2) statecoincide with energy levels of the erbium ⁴I_(11/2) state permittingenergy transfer (i.e. electron transfer) from the pumped ytterbium²F_(5/2) state to the erbium ⁴I_(11/2) state. Thus, pumping ytterbiumionic energy states provides a mechanism for populating the metastableerbium ⁴I_(13/2) state, permitting even higher levels of populationinversion and more stimulated emission than with erbium doping alone.

[0031] Ytterbium ions exhibit not only a large absorption cross-sectionbut also a broad absorption band between 900 and 1100 nm. Furthermore,the large spectral overlap between Yb³⁺ emission (²F_(7/2)-²F_(5/2)) andEr³⁺ absorption (⁴I_(15/2)-⁴I_(11/2)) results in an efficient resonantenergy transfer from the Yb³⁺ to the Er³⁺, exciting the ⁴I_(11/2) level.The energy transfer mechanism in an Yb³⁺/Er³⁺ co-doped system is similarto that for cooperative upconversion processes in an Er³⁺ doped system.However, interactions are between Yb³⁺ (donor) and Er³⁺ (acceptor) ionsinstead of between two excited Er³⁺ ions. Thus, in one embodiment thepresent invention utilizes Er:Yb co-doped boro-tellurite glass dopedwith 0.25 to 5 weight percent of an Er₂O₃ and Yb₂O₃ mixture. Typically,this glass is doped with 0.25-3 weight percent of Er₂O₃ and 0.25-3weight percent of Yb₂O₃.

[0032] As shown in FIG. 5, a boro-tellurite EDFA 50 includes aboro-tellurite active fiber 52 of the type just described and a 980 nmoptical pump 54. The active fiber is coupled between a pair of input andoutput fibers, 53 a and 53 b, typically passive double-clad silicafiber. Coupling of the signal between fibers can be achieved usingfree-space optics or by fusion splicing. Optical pump 54 can be either asingle-mode or a multi-mode pump and is coupled into the active fiberusing a pump coupler 55 such as a WDM, a side-coupler such as Goldberg'sV-groove as described in U.S. Pat. No. 5,854,865 or by using a totalinternal reflection (TIR) coupler as described in co-pending U.S. patentapplication Ser. No. 09/943,257 entitled “Total Internal Reflection(TIR) Coupler and Method for Side-Coupling Pump Light into a Fiber”,which is hereby incorporated by reference. The optical pump excites theionic rare-earth dopants in the core of the fiber to produce stimulatedemission and amplification of an input signal propagating through thefiber. Using the boro-tellurite fiber of the present invention, the EDFAprovides a moderate amount of gain over a wide bandwidth.

Experimental Procedure

[0033] The development of the boro-tellurite glasses provided in FIGS.2, 3 and 4 are the result of considerable experimentation and analysisto determine appropriate compositions that not only increase phononenergy but do so without reducing the bandwidth or deteriorating theoptical, thermal or chemical durability properties of the glass.

[0034] The different glass compositions were prepared according to thefollowing procedure: high-purity oxides (99.999% and 99.99% pure) wereweighed according to desired oxide molar percentages and mixed. Eachmixture of powders was heated in a furnace at temperatures ranging from700° C. to 800° C. depending on the melting properties of eachcomposition. The melted bath (or glass, or mixture) was then kept undera flow at 10 LPM (liter per minute) of nitrogen gas. This treatmentremoves hydroxyl impurities (OH⁻) from the glass, which are known toreduce the light-emitting properties of the Er ions. After thistreatment, the melts were cast into moulds preheated at the glasstransition temperature of each composition and the solids were annealedat this temperature for two hours before being cooled down slowly toroom temperature over a period of 15 hours.

[0035] The determination of the glass transition temperature for eachcomposition was carried out by differential scanning calorimetry. Forthese experiments, glasses were first reduced to powder and placed intoalumina crucibles and heated at a rate of 10° C./min from roomtemperature to 800° C. under a flow of nitrogen gas at 0.1 LPM. Othertemperatures, including the onset crystallization temperature Tx, thecrystallization peak temperature Tc, and the melting point temperatureTm were determined following the same procedure.

[0036] To test the chemical durability of each glass composition,samples with dimensions of approximately 4×16×24 mm were cut, polished,and weighed carefully. Then the samples were immersed in boiling waterfor fixed time periods and carefully weighed again between successiveimmersions. An important weight loss (measured in units of mg/mm²)following immersion in boiling water is indicative of poor chemicaldurability and vice versa.

[0037] Other characterization experiments included density measurementsin which the volume of each sample was determined by immersing thesamples in carbon tetrachloride, optical spectroscopy in the UV, visibleand near infra-red, and refractive index measurements using amultiwavelength prism coupler. For the determination of phonon energiesinfrared spectroscopy was performed. For these experiments, 1 mg ofglass powder was mixed with 150 mg of dried KBr and the mixture wasformed into a flat pellet by compression. During the experiments, thespectrophotometer was purged by dried air.

[0038] For the measurement of the rare-earth emission spectrum andfluorescence lifetime, 300 μm-thick samples with polished facets wereprepared. Emission spectra were recorded with a spectrometer while thesamples were pumped at 980 nm using a cw Ti:sapphire laser. Absolutevalues of the absorption and emission cross sections were calculatedusing McCumber theory. The fluorescence lifetime of Er³⁺ was determinedfrom the measured fluorescence decay curve of the ⁴I_(13/2)→⁴I_(15/2)transition.

Experimental Results

[0039] Table 1 provides a list of glasses with their respectivecomposition that were fabricated and tested. In this example, the Er₂O₃concentration was fixed at one weight percent of the total weight of theglass. For comparison, a glass containing tungsten oxide was alsoprepared.

[0040] When the Na₂O concentration is decreased in the boro-telluriteglasses, especially when Na₂O is replaced by Al₂O₃, their color changesfrom yellow to pink. Since the glasses contain erbium ions, and sincethese ions are known to confer a pink color to a transparent glassmatrix, it can be deduced that the decrease of Na₂O, especially when itis replaced by Al₂O₃, shifts the UV absorption edge towards the shorterwavelength. TABLE 1 Glass names and compositions. Glass Composition (%mol) 25Na 25 Na₂O—15 B₂O₃—60 TeO₂—1_(WT) Er₂O₃ 5Te 20 Na₂O—15 B₂O₃—65TeO₂—1_(WT) Er₂O₃ 5Ge 20 Na₂O—5 GeO₂—15 B₂O₃—60 TeO₂—1_(WT) Er₂O₃ 7Ge 18Na₂O—7 GeO₂—15 B₂O₃—60 TeO₂—1_(WT) Er₂O₃ 5Ge2Te 18 Na₂O—5 GeO₂—15B₂O₃—62 TeO₂—1_(WT) Er₂O₃ 5Al 20 Na₂O—5 Al₂O₃—15 B₂O₃—60 TeO₂—1_(WT)Er₂O₃ 7Al 18 Na₂O—7 Al₂O₃ 15 B₂O₃—60 TeO₂—1_(WT) Er₂O₃ 10Al 15 Na₂O—10Al₂O₃—15 B₂O₃—60 TeO₂—1_(WT) Er₂O₃ 5A12Te 18 Na₂O—5 Al₂O₃—15 B₂O₃—62TeO₂—1_(WT) Er₂O₃ 5Al 5Ge 15 Na₂O—5 Al₂O₃—5 GeO₂—15 B₂O₃—60 TeO₂—1_(WT)Er₂O₃ 10Al 5Ge 10 Na₂O—10 Al₂O₃—5 GeO₂—15 B₂O₃—60 TeO₂—1_(WT) Er₂O₃15K25W 15 K₂O—25 WO₃—60 TeO₂—1_(WT) Er₂O₃

[0041] Thermal Properties

[0042] An important characteristic that defines a good glass is itsresistance to crystallization. Crystallization can occur when the glassis heated above its glass transition temperature and leads to anexothermic peak in a differential scanning calorimetry (DSC) curve. SuchDSC curves are shown in FIGS. 6 and 7.

[0043] In FIG. 6, the curves 60, 61, 62 and 63 correspond respectivelyto the glasses 25Na, 5Te, 5Al, and 5Ge listed in Table 1. The firstdecrease in heat flow 65 corresponds to the glass transitiontemperature. This decrease is followed by an exothermic peak 66 that isindicative of crystallization. All the curves in FIG. 6 exhibit suchcrystallization peaks, indicating that the glass compositions 25Na, 5Te,5Ge, and 5Al do not exhibit very good thermal properties In contrast,the DSC curves 70, 71, and 72 shown in FIG. 7, correspondingrespectively to the glasses 10Al5Ge, 10Al and 5Al5Ge listed in Table 1,do not show exothermic peaks and are indicative of excellent thermalproperties. The lack of strong exothermic peaks in any of these curvesindicates that these glasses do not crystallize when heated. Suchproperties are highly desirable when the glasses are to be drawn intofibers. The characteristic temperatures for all glasses are reported inTable 2. Except for the glass compositions 25Na, 5Te, 5Ge, and 5Al, allthe other glass compositions described in Table 1 lack a crystallizationsignature upon heating in DSC curves. This illustrates the excellentthermal properties of the glass compositions of the present invention.TABLE 2 Characteristic temperatures for all glasses. Tg (glass) − GlassTg (° C.) Tx (° C.) Tc (° C.) Tm (° C.) Tg (25 Na) 25Na 279 365    s 397522 0 5Te 292 370    s 412 553 13 5Ge 310 380    vs 432  541 31 7Ge 323″ ″  s 533 44 5Ge2Te 314 ″ ″ 534 35 5Al 325 398    vs 437  537 46 7Al342 ″ ″ 549 63 10Al 362 ″ ″ — 83 5A2Te 334 ″ ″ 546 55 5Al5Ge 344 ″ ″ vs520 65 10Al5Ge 375 ″ ″  s 551 96 15K25W 352 ″ ″ vs 537 —

[0044] Chemical Durability

[0045] For optical communication applications, EDFAs must resist thenatural corrosion exercised by the environment for several decades. Thechemical durability of the glasses was tested in boiling water. Thefaster the glass looses weight in boiling water, the worse is itschemical durability. FIGS. 8, 9 and 10 represent the weight loss perunit of surface as a function of immersion time in boiling water for allthe glass compositions listed in Table 1. For clarity, these curves havebeen split into three figures. The glasses (25Na 80, 7Ge 81, 5Ge2Te 82,5Te 83, 5Ge 84, and 5Al 85) with the highest weight losses are presentedin FIG. 8, those glasses (5Al 85, 5Al2Te 86, 5Al5Ge 87, 7Al 88, and15K25W 89) with medium weight losses in FIG. 9, and those glasses(15K25W 89, 10Al5Ge 90, and 10Al 91) with the lowest weight losses inFIG. 10. For ease of comparison, the curve obtained for the glass 5Al 85is repeated in FIG. 9. Likewise that of the glass 15K25W 89 is repeatedin FIG. 10.

[0046] The glass composition with the highest alkali metal oxidecontent, 25Na, has the lowest chemical durability. After one hour inboiling water, the glass lost 1.74 mg/mm². Glasses containing highalkali metal oxide concentrations have the tendency to exhibit a higherdecomposition rate in water. Alkali metal ions favor the penetration ofwater in the glass that leads to a decomposition of the glass network.Therefore a lower concentration of Na₂O in the glass results in a betterchemical durability. As shown in FIG. 10, when the Al₂O₃ concentrationis 10%, glasses 10Al 91 and 10Al5Ge 90 show good chemical durability.Their respective weight losses after one hour of immersion in boilingwater are 0.00255 mg/mm² and 0.00485 mg/mm².

[0047] Density and Refractive Index

[0048] Table 3 summarizes the weight density and the refractive indexmeasured at several wavelengths (633, 830, 1307, and 1550 nm) for allthe glass compositions reported in Table 1. The refractive indexincreases with the density of the glass. The introduction of Al₂O₃ tendsto decrease the density of the glass and consequently its refractiveindex. When the modifier Na₂O is replaced by the intermediate Al₂O₃ thenumber of non-bridging oxygen decreases. In oxide glasses, the ionicrefractivity of bridging oxygen is smaller than the ionic refractivityof non-bridging oxygen. So the replacement of Na₂O by Al₂O₃ leads to adecrease of the refractive index. Another contribution might beattributed to an increased polarizability of Na⁺ ions versus Al³⁺because their ionic radius is larger.

[0049] The density of the glass decreases when Na₂O is replaced byAl₂O₃. While the glass modifier Na₂O fills the cavities of thepreexisting glass structure, the intermediate Al₂O₃ participates andchanges the network of the glass. Hence, Al₂O₃ expands the volume of theglass network. Since Na⁺ and Al³⁺ have the same weight, the volumeincrease associated with the addition of Al₂O₃ translates into adecrease in density of the glass.

[0050] As shown in Table 3, the introduction of Ge or more Te in theglass increases the refractive index. Ge and Te, like other heavy atomsincrease the refractive index. This effect is the strongest with Te,which is heavier, larger, and consequently leads to a higherpolarizability compared with Ge. GeO₂ and TeO₂ as network formers modifythe structure of the glass and could expand its volume like Al₂O₃.However, since they are heavy atoms, the density of the glass isincreased, when used instead of Na₂O. TABLE 3 Glass density 25° C. (+/−0.01 g/cm³) and refractive index measured at several wavelengths invarious glasses. n at n at n at n at Glass d (g/cm³) 633 nm 830 nm 1307nm 1550 nm 25 Na 4.26 1.8406 1.8222 1.8066 1.801 5 Te 4.45 1.8888 1.86821.8509 1.8449 5 Ge 4.4 1.8667 1.8477 1.8313 1.8258 7 Ge 4.46 1.87281.8535 1.8372 1.8315 5 Ge 2 Te 4.47 1.893 1.8737 1.8584 1.8506 5 Al 4.241.8433 1.8254 1.8107 1.8047 7 Al 4.26 1.8384 1.8213 1.8059 1.801 10 Al4.18 1.8121 1.796 1.7822 1.7776 5 Al 2 Te 4.31 1.8594 1.8399 1.82551.8202 5 Al 5 Ge 4.34 1.8508 1.8329 1.8178 1.8127 10 Al 5 Ge 4.3 1.83861.8221 1.8076 1.8023 15 K 25 W 5.34 1.9949 1.9697 1.9481 1.9432

[0051] Phonon Energy

[0052] Boron oxide is introduced into tellurite oxide glasses toincrease the phonon energy of the lattice, while maintaining goodchemical durability of the glass. Transmission spectra in the infraredwere measured using infra-red spectroscopy to verify this effect. Theglasses tested can be classified in four groups: 1)alkali-boro-tellurite glasses; 2) alkali-boro-tellurite glassescontaining Al₂O₃ or GeO₂; 3) tellurite glasses containing some tungstenoxide; and 4) pure alkali-tellurite glasses. Table 4 summarizes theglasses tested and the measured phonon energies. TABLE 4 Absorption wavenumber (cm⁻¹) from the transmission spectra Composition of the glass (%mol) - V: curve in V shape, W: curve in W shape, Sh: shoulder, FSh: flat1_(WT) Er₂O₃ shoulder 30 Na₂O-10 B₂O₃-60 TeO₂ 1321.9 1014.4 941.1 759.0697.0 (V) (W) (W) (V) (Sh) 25 Na₂O-15 B₂O₃-60 TeO₂ 1332.8 1258.7 1029.2935.7 763.2 682.6 (V) (Sh) (Sh) (V) (FSh) (V) 20 Na₂O-20 B₂O₃-60 TeO₂1337.5 1263.1 1046.6 938.9 766.8 695.7 (V) (FSh) (Sh) (V) (Sh) (Sh) 20Na₂O-15 B₂O₃-65 TeO₂ 1331.9 1258.7 1035.5 933.3 764.5 689.0 (V) (Sh)(Sh) (V) (Sh) (Sh) 20 Na₂O-5 Al₂O₃-15 B₂O₃-60 TeO₂ 1340.0 1245.2 1020.0910.0 761.4 715.7 (W) (W) (FSh) (FSh) (Sh) (V) 20 Na₂O-5 GeO₂-15 B₂O₃-60TeO₂ 1334.2 1255.3 1035.5 934.4 764.5 685.7 (V) (Sh) (Sh) (V) (Sh) (Sh)20 K₂O-10 WO₃-10 B₂O₃-60 TeO₂ 1335.1 1248.8 1059.1 *916.8  848.7 785.9690.8 (W) (W) (Sh) (V) (FSh) (Sh) (Sh) 15 K₂O-25 WO₃-60 TeO₂ *928.6 845.5 773.0 682.1 (V) (FSh) (Sh) (Sh) 35 Na₂O-65 TeO₂ 756.4 (V) 30Na₂O-70 TeO₂ 757.9 688.5 (Sh) (Sh) Component the most responsible B₂O₃B₂O₃ B₂O₃ B₂O₃ WO₃ TeO₂ TeO₂ Vibration bond attribution ═B—O≡ ═B—O—B≡═B—O—B≡ O—Te—O O—Te—O s = stretching, s-s = symmetry or B—O—B or or[W0₄] or or stretching, as-s = asymmetry stretching, O—B—O (s) ═B—O—═B—O— (g) Te—O⁻ Te—O⁻ b = bending, g = group (as-s) (as-s) (as-s) (as)(as) Other responsible component *WO₃ B₂O₃ could influence ═B—O—B═ (b)

[0053]FIG. 11 shows the transmission spectra of two alkali-telluriteglasses 35 Na₂O−65 TeO₂−1_(WT) Er₂O₃ 100 and 30 Na₂O−70 TeO₂−1_(WT)Er₂O₃ 101. This figure clearly illustrates that the highest phononenergy of TeO₂ is around 757 cm⁻¹. The alkaline oxide, as a modifier,has no influence on the phonon energy of the lattice of the glass. Thisabsorption around 757 cm⁻¹ can be attributed to the asymmetric vibrationof the O—Te—O or O—Te—O bonds.

[0054]FIG. 12 shows the transmission spectra of alkali-boro-telluriteglasses 30 Na₂O−10 B₂O₃−60 TeO₂−1_(WT) Er₂O₃ 110, 20 Na₂O−15 B₂O₃−65TeO₂−1_(WT) Er₂O₃ 111, 25 Na₂O−15 B₂O₃−60 TeO₂−1_(WT) Er₂O₃ 112, and 20Na₂O−20 B₂O₃−60 TeO₂−1_(WT) Er₂O₃ 113. As expected, the introduction ofB₂O₃into tellurite glasses increases the phonon energy significantly upto 1337 cm⁻¹ as also reported in Table 4. This phonon energy can beattributed to the asymmetric stretching of the bonds ═B—O≡ or O—B—O.Note that the shapes of the spectra shown in FIG. 12 are nearlyidentical for all four glasses. A small difference is in the lack of ashoulder near 1260 cm⁻¹ in glass 110 containing 10% of B₂O₃. Since thisband is due to the B—O—B stretching, we suspect that 10% of B₂O₃ is notenough to have a significant proportion of two boron atoms connected tothe same oxygen, if the glass is homogeneous.

[0055]FIG. 12 also shows that the addition of boron oxide to thetellurite glasses leads to the appearance of another broad absorptionband around 937 cm⁻¹ with a shoulder at around 1033 cm⁻¹. These twobands can be attributed to the asymmetric stretching of ═B—O—B≡ or ═B—O—bonds. As expected the absorption bands due to TeO₂ at around 764 and692 cm⁻¹ are still observable.

[0056] The transmission spectra of glasses with compositions 20 Na₂O−5Al₂O₃−15 B₂O₃−60 TeO₂−1 wt. % Er₂O₃ 120 and 20 Na₂O−5 GeO₂−15 B₂O₃−60TeO₂−1 wt. % Er₂O₃ 121 shown in FIG. 13 are very similar to those inFIG. 12. The addition of 5% of germanium oxide doesn't seem to influencethe spectrum in one-way or another. In contrast, when Al₂O₃ is added toalkali-boro-tellurite glasses the transmission spectrum changes as shownin FIG. 13.

[0057] In alkali-tellurite glasses containing tungsten oxide such as inglasses with compositions 15 K₂O−25 WO₃−60 TeO₂−1_(WT) Er₂O₃ 130 and 20K₂O−10 WO₃−10 B₂O₃−60 TeO₂−1_(WT) Er₂O₃ 131, two additional absorptionbands are observed respectively at 929 and 846 cm⁻¹ as illustrated inFIG. 14. The two bands 140 and 141 are attributed to the tungsten oxideand more particularly to the vibrations of the WO₄ tetrahedrons. WhenB₂O₃ is added to a tungstate alkali-tellurite glass, the absorption bandat 1249 cm⁻¹ typical of the B—O—B stretching mode is well defined. Thisindicates that the proportion of two boron atoms connected to the sameoxygen is important and that consequently WO₃ and B₂O₃ can not be mixedwell in this particular glass composition.

[0058] As a proof, when the glass was cast at low cooling rate, theglass had poor optical transparency, indicative of phase separation andpoor homogeneity.

[0059] Spectral properties

[0060] The optical properties (absorption and emission) of the erbiumions that were doped into the different glasses are summarized in Table5. The second column of the table indicates the concentration of erbiumions. This concentration varies with the glass composition because theconstant amount of erbium oxide that was incorporated into the differentglasses was measured in wt %. The third column (∫σ_(a)(λ)dλ in cm²)describes the total absorption cross section of the 1550 nm band. Thefourth column gives the absorption bandwidth (Δλ_(a) in nm). The fifthand sixth columns describe the total emission cross section of the 1550nm band (∫σ_(e)(λX)dλ in cm²), and the emission bandwidth (Δλ_(e) innm), respectively. The last column lists the measured lifetime(τ_(meas.) in ms) of the ⁴I_(13/2) excited state. TABLE 5 Opticalproperties of Er³⁺ ions contained in various glasses. τ_(meas.) Er³⁺∫σ_(a)(λ)dλ ∫σ_(e)(λ)dλ Δλ_(e meas.) (+/− 0.1 ms) at Glass (10²⁰ions/cm³) (10⁻¹⁹ cm²) Δλ_(a) (nm) (10⁻¹⁹ cm²) Δλ_(e) (nm) (nm) 1535 nm25 Na 1.328 4.47 57.28 4.53 54.47 54.30 3.17 5 Te 1.387 4.84 59.33 5.0857.86 56.01 2.98 5 Ge 1.372 5.07 59.03 5.28 57.40 55.80 2.97 7 Ge 1.3894.83 60.45 5.16 59.91 57.48 3.26 5 Ge 2 Te 1.392 4.68 60.06 4.62 58.6655.22 3.41 5 Al 1.32 4.74 59.32 4.91 57.48 56.21 2.93 7 Al 1.327 4.3960.52 4.59 58.88 55.97 2.81 10 Al 1.301 4.68 62.84 4.94 61.90 61.37 3.075 Al 2 Te 1.342 4.78 60.49 4.93 59.02 59.25 2.96 5 Al 5 Ge 1.352 4.8761.68 5.13 60.04 57.06 3.20 10 Al 5 Ge 1.339 4.93 66.81 5.32 66.23 61.992.05 15 K 25 W 1.665 5.12 61.29 5.41 60.12 55.75 3.66

[0061] Gain Properties

[0062] To evaluate the optical gain properties of Er³⁺ dopedboro-tellurite glasses for use in optical amplifiers and lasers, opticalfibers were fabricated from these glasses and tested. The preform forthe fiber was fabricated from the following glasses: for the core aglass with composition 60TeO₂+15B₂O₃+10Al₂O₃+15Na₂O+0.5 wt. % Er₂O₃ wasused, and for the cladding a glass with composition57.75TeO₂+15B₂O₃+10Al₂O₃+15Na₂O+2.25 ZnO. At the wavelength of 1550 nm,the refractive index of the core and cladding were n=1.7834 andn=1.7738, respectively. The fiber was drawn from the preform usingstandard fiber pulling techniques.

[0063]FIG. 15 shows the gain 150 and noise FIG. 151 measured in a 15cm-long tellurite fiber for an input signal at 1535 nm and as a functionof the power of the pump with wavelength 976 nm. A gain of 12 dB wasachieved at a pumping power of 110mw and the corresponding noise figurewas around 5 dB. The gain curve indicates that the saturation is notreached. The gain can be further raised by increasing the pumping poweror by increasing the fiber length.

[0064]FIG. 16 is the gain spectrum 160 measured in the same telluritefiber at a pumping power of 112 mW. A maximum gain of 13.5 dB isachieved at 1533 nm and a gain of 2 dB is measured at longer wavelengthnear 1600 nm. These results show that these new glasses can provide gainover a broad spectrum, especially at longer wavelengths

[0065] While several illustrative embodiments of the invention have beenshown and described, numerous variations and alternate embodiments willoccur to those skilled in the art. Such variations and alternateembodiments are contemplated, and can be made without departing from thespirit and scope of the invention as defined in the appended claims.

We claim:
 1. A boro-tellurite glass composition comprising the following ingredients: TeO₂ from 50 to 70 mole percent, B₂O₃ from 5 to 22 mole percent, R₂O from 5 to 25 mole percent, MO from 0 to 15 mole percent, A₂O₃ from 5 to 18 mole percent, GeO₂ from 0 to 7 mole percent, and L₂O₃ from 0.25 to 10 weight percent, wherein R₂O is selected from the oxides Li₂O, K₂O, Na₂O and mixtures thereof, MO is selected from the oxides BaO, MgO, CaO, ZnO and mixtures thereof, A₂O₃ is selected from Al₂O₃, Y₂O₃ and mixtures thereof, and L₂O₃ is selected from rare earth oxides Er₂O₃, Yb₂O₃, Tm₂O₃, Tb₂O₃, CeO₂, Sm₂O₃ and Nd₂O₃ and mixtures thereof.
 2. The boro-tellurite glass composition of claim 1, wherein the composition comprises TeO₂ from 55 to 65 mole percent, B₂O₃ from 10 to 20 mole percent, A₂O₃ from 7 to 15 mole percent, a glass network modifier R₂O from 10 to 20 mole percent, a glass network modifier MO from 0 to 10 mole percent, GeO₂ from 0 to 5 mole percent and rare-earth dopant L₂O₃ from 0.25 to 6 weight percent.
 3. The boro-tellurite glass of claim 2, wherein A₂O₃ is 7 to 15 mole percent Al₂O₃.
 4. The boro-tellurite glass of claim 2, wherein A₂O₃ is 10 to 15 mole percent Al₂O₃.
 5. The boro-tellurite glass composition of claim 2, wherein the composition comprises approximately 60 mole percent TeO₂, approximately 15 mole percent B₂O₃, approximately 10 mole percent Al₂O₃, and approximately 15 mole percent Na₂O.
 6. The boro-tellurite glass composition of claim 2, wherein the rare-earth dopant L₂O₃ comprises approximately 0.25 to 3 weight percent of Er₂O₃.
 7. The boro-tellurite glass composition of claim 2, wherein the rare-earth dopant L₂O₃ comprises a mixture of approximately 0.25 to 5 weight percent of Er₂O₃ and Yb₂O₃.
 8. The boro-tellurite glass composition of claim 7, wherein the mixture comprises 0.25 −3 weight percent Er₂O₃ and 0.25 −3 weight percent Yb₂O₃.
 9. The boro-tellurite glass composition of claim 2, wherein the rare-earth dopant L₂O₃ comprises approximately 0.25 to 3 weight percent of Tm₂O₃.
 10. A boro-tellurite glass composition comprising the following ingredients: TeO₂ from 55 to 65 mole percent, B₂O₃ from 10 to 20 mole percent, Na₂O from 10 to 20 mole percent, MO from 0 to 10 mole percent, Al₂O₃ from 7 to 15 mole percent, GeO₂ from 0 to 7 mole percent, and L₂O₃ from 0.25 to 6 weight percent, wherein MO is selected from the oxides BaO, MgO, CaO, ZnO and mixtures thereof, and L₂O₃ is selected from rare earth oxides Er₂O₃, Yb₂O₃, Tm₂O₃, Tb₂O₃, CeO₂, Sm₂O₃ and Nd₂O₃ and mixtures thereof.
 11. The boro-tellurite glass of claim 10, comprising 10 to 15 mole percent Al₂O₃.
 12. The boro-tellurite glass composition of claim 10, wherein the rare-earth dopant L₂O₃ comprises approximately 0.25 to 3 weight percent of Er₂O₃.
 13. The boro-tellurite glass composition of claim 10, wherein the rare-earth dopant L₂O₃ comprises a mixture of approximately 0.25 to 5 weight percent of Er₂O₃ and Yb₂O₃.
 14. The boro-tellurite glass composition of claim 10, wherein the mixture comprises 0.25 −3 weight percent Er₂O₃ and 0.25 −3 weight percent Yb₂O₃.
 15. An optical fiber, comprising a core and a cladding formed of a glass having the following ingredients: TeO₂ from 50 to 70 mole percent, B₂O₃ from 5 to 22 mole percent, R₂O from 5 to 25 mole percent, MO from 0 to 20 mole percent, A₂O₃ from 5 to 18 mole percent, GeO₂ from 0 to 7 mole percent, wherein R₂O is selected from the oxides Li₂O, K₂O, Na₂O and mixtures thereof, MO is selected from the oxides BaO, MgO, CaO, ZnO and mixtures thereof, A₂O₃ is selected from Al₂O₃, Y₂O₃ and mixtures thereof, and said core further comprising, Rare-earth dopant L₂O₃ from 0.25 to 10 weight percent, wherein L₂O₃ is selected from rare earth oxides Er₂O₃, Yb₂O₃, Tm₂O₃, Tb₂O₃, CeO₂, Sm₂O₃ and Nd₂O₃ and mixtures thereof.
 16. The optical fiber of claim 15, wherein the composition comprises TeO₂ from 55 to 65 mole percent, B₂O₃ from 10 to 20 mole percent, A₂O₃ from 7 to 15 mole percent, a glass network modifier R₂O from 10 to 20 mole percent, a glass network modifier MO from 0 to 10 mole percent, GeO₂ from 0 to 5 mole percent and rare-earth dopant L₂O₃ from 0.25 to 6 weight percent.
 17. The optical fiber of claim 16, wherein A₂O₃ is 10 to 15 mole percent Al₂O₃.
 18. The optical fiber of claim 16, wherein the rare-earth dopant L₂O₃ comprises approximately 0.25 to 3 weight percent of Er₂O₃.
 19. The optical fiber of claim 16, wherein the rare-earth dopant L₂O₃ comprises a mixture of approximately 0.25 to 5 weight percent of Er₂O₃ and Yb₂O₃.
 20. An erbium doped fiber amplifier (EDFA), comprising: A fiber having a core and a cladding formed from a glass having the following ingredients: TeO₂ from 50 to 70 mole percent, B₂O₃ from 8 to 22 mole percent, R₂₀ from 5 to 20 mole percent; MO from 0 to 20 mole percent, A₂O₃ from 7 to 18 mole percent, GeO₂ from 0 to 7 mole percent, wherein R₂O is selected from the oxides Li₂O, K₂ 0, Na₂O and mixtures thereof, MO is selected from the oxides BaO, MgO, CaO, ZnO and mixtures thereof, A₂O₃ is selected from Al₂O₃, Y₂O₃ and mixtures thereof, said core further comprising rare-earth dopant L₂O₃ from 0.25 to 10 weight percent, wherein L₂O₃ is selected from rare earth oxides Er₂O₃, Yb₂O₃, Tm₂O₃, Tb₂O₃, CeO₂, Sm₂O₃ and Nd₂O₃ and mixtures thereof; and A 980 nm optical pump configured to pump the ionic energy levels of the rare-earth dopant in said fiber to produce stimulated emission and amplification of an input signal propagating through said fiber.
 21. The EDFA of claim 20, wherein the composition comprises TeO₂ from 55 to 65 mole percent, B₂O₃ from 10 to 20 mole percent, A₂O₃ from 7 to 15 mole percent, a glass network modifier R₂O from 10 to 20 mole percent, a glass network modifier MO from 0 to 10 mole percent, GeO₂ from 0 to 5 mole percent and rare-earth dopant L₂O₃ from 0.25 to 6 weight percent.
 22. The EDFA of claim 21, wherein A₂O₃ is 10 to 15 mole percent Al₂O₃.
 23. The EDFA of claim 21, wherein the rare-earth dopant L₂O₃ comprises approximately 0.25 to 3 weight percent of Er₂O₃.
 24. The EDFA of claim 21, wherein the rare-earth dopant L₂O₃ comprises a mixture of approximately 0.25 to 5 weight percent of Er₂O₃ and Yb₂O₃.
 25. The EDFA of claim 21, wherein optical pump comprises a multi-mode pump and a pump coupler.
 26. The EDFA of claim 25, wherein the pump coupler comprises a TIR coupler. 